From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta0.migadu.com (out-159.mta0.migadu.com [91.218.175.159]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0D3C8492E53 for ; Wed, 9 Sep 2026 02:58:05 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.159 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788922688; cv=none; b=he7vY4IahC/f2cnKTklJOmxAPOcQOr5ODBXw4bm2W2+VV2yAPCcpkrNqqW3zDmtRrnLXbmjyRnso9VC5daAA9J/GS51b7rDkjhxn+vhlD/1Bwf9vXk5EiBhK+vxjYbqlLCDj8M5vhph2L2wxkaqgtF7QrwRlF6h8H6Ene03HLTY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788922688; c=relaxed/simple; bh=lkc5mj4lW16ivPJ8rrhfx1h8FyU/NRPzkzLjWPo4X0M=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=T7Xsbn4NHg/Up/VH6V8fBzqHm5fUytq2J1SQxgN3LVY0ebwAi+5FpWNw/nLwqpXjZ/3w4NiS2Vxx1jNEXW7pb60XeyjLAB5tTUqj66aDFB7fVNqPhLz440n97YhGV8Ia0FTpaVPb48sn4QZactQOsv0zLtBf384NGCE2yXwZqiY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=OWYdSwjV; arc=none smtp.client-ip=91.218.175.159 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="OWYdSwjV" X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=lkc5mj4lW16ivPJ8rrhfx1h8FyU/NRPzkzLjWPo4X0M=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1788922682; v=1; x=1789527482; b=OWYdSwjVUpqSs+Vn9TqxkwcuUFMkZZ1dLMwV2lC7v0Wm684n7rs+xina7S/m4Lxf125Q00gf SSdpynspMaDujlnhBGp2zpa8Cx2z1IF7xKCT/BYXsBtBAc1yjeZYuCGXQh8/ntHwaukQ66H88Et aXjL34Q8Z6T/ku2aDa4gc13s= X-Envelope-To: linux-kernel@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id b0e8a5b124b5113c; Wed, 09 Sep 2026 02:58:02 +0000 X-Mizu-Trace-ID: b0e8a5b124b5113c X-Migadu-Flow: FLOW_OUT Message-ID: <8bbee583-6f21-4817-bfeb-2d60057380a3@linux.dev> Date: Wed, 9 Sep 2026 10:57:34 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net v2] tcp: fix data-race in tcp_recv_should_stop To: quanyeyang@proton.me, Eric Dumazet , Neal Cardwell , Kuniyuki Iwashima , "David S. Miller" , Jakub Kicinski , Paolo Abeni , Simon Horman , "Matthieu Baerts (NGI0)" , Geliang Tang , Mat Martineau Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org References: <20260909-mptcp-sk-err-net-v2-1-5044abecac90@proton.me> From: Jiayuan Chen In-Reply-To: <20260909-mptcp-sk-err-net-v2-1-5044abecac90@proton.me> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 9/9/26 10:05 AM, Quanye Yang via B4 Relay wrote: > From: Quanye Yang > > BUG: KCSAN: data-race in do_recvmmsg / mptcp_recvmsg > > read-write (marked) to 0xffff8880134d391c of 4 bytes by task 2619 on cpu 1: > instrument_atomic_read_write include/linux/instrumented.h:113 [inline] > sock_error include/net/sock.h:2565 [inline] > do_recvmmsg+0x50c/0x580 net/socket.c:3049 > __sys_recvmmsg net/socket.c:3144 [inline] > __do_sys_recvmmsg net/socket.c:3167 [inline] > __se_sys_recvmmsg net/socket.c:3160 [inline] > __x64_sys_recvmmsg+0x161/0x180 net/socket.c:3160 > x64_sys_call+0x19c7/0x1ca0 arch/x86/include/generated/asm/syscalls_64.h:300 > do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline] > do_syscall_64+0xde/0x3d0 arch/x86/entry/syscall_64.c:84 > entry_SYSCALL_64_after_hwframe+0x77/0x7f > > read to 0xffff8880134d391c of 4 bytes by task 2620 on cpu 0: > tcp_recv_should_stop include/net/tcp.h:3086 [inline] > mptcp_recvmsg+0x54d/0xd50 net/mptcp/protocol.c:2466 > inet_recvmsg+0x204/0x210 net/ipv4/af_inet.c:894 > sock_recvmsg_nosec net/socket.c:1151 [inline] > sock_recvmsg+0x11a/0x140 net/socket.c:1173 > ____sys_recvmsg+0x14b/0x3c0 net/socket.c:2933 > ___sys_recvmsg+0x116/0x160 net/socket.c:2975 > __sys_recvmsg net/socket.c:3008 [inline] > __do_sys_recvmsg net/socket.c:3014 [inline] > __se_sys_recvmsg net/socket.c:3011 [inline] > __x64_sys_recvmsg+0xeb/0x160 net/socket.c:3011 > x64_sys_call+0x1319/0x1ca0 arch/x86/include/generated/asm/syscalls_64.h:48 > do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline] > do_syscall_64+0xde/0x3d0 arch/x86/entry/syscall_64.c:84 > entry_SYSCALL_64_after_hwframe+0x77/0x7f > > value changed: 0x0000006b -> 0x00000000 > > Reported by Kernel Concurrency Sanitizer on: > CPU: 0 UID: 0 PID: 2620 Comm: syz.2.33 Not tainted 7.2.0-g39d4f32c5d53 #76 PREEMPT(full) > Hardware name: QEMU Ubuntu 26.04 PC (i440FX + PIIX, 1996), BIOS 1.17.0-debian-1.17.0-1ubuntu1 04/01/2014 > > do_recvmmsg() calls sock_error() without the socket lock. sock_error() > clears sk_err with xchg(), which races with the unmarked load in > tcp_recv_should_stop(). The same race exists for plain TCP with > concurrent recvmmsg() readers; KCSAN reported it on an MPTCP socket > because mptcp_recvmsg() uses this helper. > > Use READ_ONCE() for the lockless peek. No extra ordering is needed: > the value is only used to decide whether receiving should stop. > This does not consume sk_err; the check-then-sock_error() TOCTOU > on the no-data paths is a separate issue. > > Fixes: eb477fdd6803 ("tcp: add recv_should_stop helper") Is the Fixes tag right? eb477fdd6803 only moved the existing check into a helper, the plain read was already there before. > Reported-by: Matthieu Baerts (NGI0) > Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/632 > Signed-off-by: Quanye Yang > --- > Link: https://lore.kernel.org/all/14749060-d011-41e7-9a4b-754eb5bd9d5b@redhat.com/ > --- > Changes in v2: > - add Reported-by and Closes for the MPTCP syzkaller report > - Link to v1: https://patch.msgid.link/20260908-mptcp-sk-err-net-v1-1-da71aaec9afd@proton.me > --- > include/net/tcp.h | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/include/net/tcp.h b/include/net/tcp.h > index 436495ff2271..c61d8678eafd 100644 > --- a/include/net/tcp.h > +++ b/include/net/tcp.h > @@ -3082,7 +3082,8 @@ enum skb_drop_reason tcp_inbound_hash(struct sock *sk, > > static inline int tcp_recv_should_stop(struct sock *sk) > { > - return sk->sk_err || > + /* sk_err can be cleared locklessly by sock_error(). */ > + return READ_ONCE(sk->sk_err) || > sk->sk_state == TCP_CLOSE || > (sk->sk_shutdown & RCV_SHUTDOWN) || > signal_pending(current); The lockless writers are sock_error() from SO_ERROR and recvmmsg(), so every plain read of sk_err on a TCP socket has the same race, not only this helper:     tcp_recvmsg_locked     tcp_sendmsg_locked     tcp_splice_read     sk_stream_wait_memory     tcp_bpf_sendmsg     tcp_bpf_recvmsg_parser     mptcp_splice_read     mptcp_recvmsg I'd drop the Fixes tag, target net-next and annotate all of them like e13ec3da05d1, instead of pointing at a MPTCP commit. Eric, Matthieu, does that sound OK?